There is a clear trend towards using higher carrier frequencies and high date rates in optical communications systems. One such arrangement employs Mach Zehnder modulators supplied with individual light, i.e., optical carrier, signals to be modulated. Each of the Mach Zehnder modulators is modulated directly by a microwave sub-carrier that has been modulated with the desired digital data. See for example, an article authored by W. D. Jemison et al., entitled “Microwave Photonic Vector Modulator for High-Speed Wireless Digital Communications”, IEEE Microwave And Wireless Components Letters, pp 125–127, Vol. 12, No. 4, April 2002. This and other known optical signal modulation arrangements have limited bandwidth and, hence, there is a limit on the data rate that may be used in optical communications systems employing such modulation schemes.